Continuous queries over data streams may suffer from blocking operations and/or unbound wait, which may delay answers until some relevant input arrives through the data stream. These delays may turn answers, when they arrive, obsolete to users who sometimes have to make decisions with no help whatsoever. Therefore, it can be useful to provide hypothetical answers - "given the current information, it is possible that X will become true at time t" - instead of no information at all. In this paper we present a semantics for queries and corresponding answers that covers such hypothetical answers, together with an online algorithm for updating the set of facts that are consistent with the currently available information.
翻译：对数据流的不断查询可能因阻断操作和/或无限制的等待而受到影响,这可能推迟答复,直到某些相关输入通过数据流。这些延迟可能使答案在到达时被用户所过时,用户有时不得不在没有任何帮助的情况下作出决定。因此,提供假设答案是有用的——“如果提供目前的信息,X有可能在时间t变成真实”,而不是完全没有信息。在本文件中,我们提出了一个查询的语义和相应的答案,涵盖这些假设答案,以及更新符合现有信息的整套事实的在线算法。
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Source: Apple - iOS 8
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